1. Field of the Invention
The invention relates to an FFT-size detection method and a cell search method based on FFT-size detection, and more particularly to a cell search method based on FFT-size detection in cellular system.
2. Description of the Related Art
In a cellular communication system, a subscriber station (SS) may receive downlink-transmitted signals from different cells or base stations, in response to subscriber station activation and may need to differentiate these signals using different cell codes or base station ID (BS ID). For example, in an OFDM-CDMA cellular system, the downlink-transmitted signals from different cells are differentiated using scrambling codes (cell codes), thereby allowing for reuse of frequency and spreading codes in contiguous cells. As such, an SS terminal, when switched on, locates a cell (i.e., synchronizing to the associated downlink scrambling code) before any communication. This procedure is known as initial cell search. During active or idle modes of an SS terminal, cell searching is also required to identify handoff candidates. This procedure is known as target cell search. The performance of cell search directly impacts the perceived switch-on delay, link quality and power consumption of an SS. Therefore, cell search is important for the design of OFDM based communication systems.
FIG. 1 shows the frame structure of the SCH-based cell search method. Each frame consists of M OFDM symbols. Each OFDM symbol comprises NFFT sample useful data and NGI sample cyclic prefix (CP, or called guard interval, GI) data for avoiding inter symbol interference (ISI) as well as inter-carrier interference (ICI). Accordingly, the length of an OFDM symbol, NOFDM, is the sum of the length of useful data and cyclic prefix data. The downlink-transmitted signal in FIG. 1 includes common pilot channel (CPICH) signal, synchronization channel (SCH) signal, and traffic channel (TCH) signal. CPICH signal contains information about the scrambling code, and SCH signal about the group code and frame timing. TCH signal is used for transmitting TCH data.
In the receiver of an SS, the received signal is processed by the cell search procedure shown in FIG. 2. The procedure involves symbol synchronization to detect OFDM symbol timing (OFDM symbol boundary), frame synchronization and group identification to detect frame timing (frame boundary) and the group code, and scrambling-code identification to detect the scrambling code. In symbol synchronization, symbol timing is detected using the correlation property of CP. In frame synchronization and group identification, after removing GI from the received signal and performing N_FFT point discrete Fourier transform (DFT) (or, more efficiently, fast Fourier transform (FFT)), the frame timing and group code are simultaneously detected using SCH signal in frequency domain. In group identification, the scrambling code is identified from the detected group using CPICH signal, and verification is conducted to avoid false detection, thereby minimizing unnecessary MS activities.